You'll inevitably work with dates and time at some point in your career. Whether you're building a countdown timer for a product launch, or scheduling some task for the computer to execute in the future, like sending out email notifications to your users, dates and time will come up in unexpected ways.
You'll learn techniques and best practices that will make these kinds of tasks easy for you in the future.
More imporantly, however, you'll learn skills that are common in many different kinds of web development situations, like the
setInterval method, as well as event handlers, loops, the DOM (Document Object Model), and more.